Mineral precipitation methods

ABSTRACT

The present invention provides methods for mineral precipitation of porous particulate starting materials using isolated urease.

CROSS-REFERENCE

This application is a U.S. national phase of International Application No. PCT/US2014/062557, filed on Oct. 28, 2014, which claims priority to U.S. Provisional Application No. 61/896,340, filed Oct. 28, 2013, and U.S. Provisional Application No. 61/916,908, filed Dec. 17, 2013, all of which are incorporated by reference herein in their entirety.

STATEMENT OF U.S. GOVERNMENT INTEREST

This invention was made with government support under grant number 1233658 awarded by the National Science Foundation and grant number 0856801 by the National Science Foundation. The government has certain rights in the invention.

SUMMARY OF THE INVENTION

The present invention provides mineral precipitation methods, comprising

combining a porous starting material with

-   -   (i) a source of urease;     -   (ii) urea; and     -   (iii) a source of divalent cations;

wherein (i), (ii), (and (iii) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate mineral precipitation of the starting material to produce a starting material complex; and

(b) contacting the starting material complex with a base under conditions to mitigate formation of ammonium salts in the starting material complex. In one embodiment, the base is introduced into the starting material complex at one or more locations. In another embodiment, contacting the starting material complex with the base results in an increased pH of the starting material complex at the site of the contacting. In various further embodiments, the base is selected from the group consisting of NaOH, KOH, Ca(OH)₂, Mg(OH)₂, and combinations thereof. In one embodiment, the source of urease comprises isolated urease or cells expressing urease.

In various non-limiting embodiments, the method can be used for one or more of improving bearing capacity of foundations; reducing settlement potential of foundations or embankments; stabilizing slopes; reducing the potential for triggering earthquake-induced liquefaction; reducing the potential for triggering static liquefaction; mitigating the potential for damaging ground displacements subsequent to triggering of liquefaction; increasing lateral resistance of foundations; enhancing stability of slopes or embankments; lateral earth pressures on retaining walls; increasing passive resistance of retaining walls; increasing capacity of ground anchors or soil nails; increasing the side resistance and tip resistance of deep foundations; facilitating tunneling in running or flowing ground; stabilizing excavation bottoms; soil erosion control; and groundwater control.

In various further embodiments, the starting material can be selected from the group consisting of sand, silt, clay, other sediments, sawdust, igneous rocks, metamorphic rocks, gravel, fractured crystalline rocks, cracked concrete and sedimentary rocks including but not limited to conglomerate, breccia, sandstone, siltstone, shale, limestone, gypsum, and dolostone, and combinations thereof. In other embodiments, the source of divalent cations is selected from the group consisting of calcium, magnesium, strontium, and other cations in the divalent state such as iron, cadmium, manganese, lead, or zinc ions, or combinations thereof. In a further embodiment, the method further comprises introducing a clay slurry into the starting material prior to or concurrent with combining the starting material with the source of urease, the urea, and the source of divalent cations.

DESCRIPTION OF THE FIGURES

FIG. 1. LV-Scanning electron microscope images of a) well-grown and cementing calcite crystals; b) cementing calcite crystals at inter-particle contact; c) indention of quartz surface (arrows) and nucleation of calcite crystals (red arrows); d) calcite crystal growing on quartz surface.

FIG. 2. P-q plot failure envelopes for 20-30 silica sand: ▪Cemented (D_(r)=60%); ∘ Uncemented (D_(r)=60%).

FIG. 3. P-q plot failure envelopes for F-60 silica sand: ▪Cemented (D_(r)=35%); ∘ Uncemented (D_(r)=37%).

FIG. 4. Results of triaxial compression tests performed using F-60 Ottawa sand (medium sand).

FIG. 5. Results of triaxial compression tests performed using medium sand w/bentonite slurry.

FIG. 6. Results of triaxial compression tests performed using F-60 fine sand.

FIG. 7. Image from mix & compact columns using silica 20-30. These are silica sand particles variously covered with CaCO₃ and cemented at the inter-particle contacts.

FIG. 8. Image from same mix & compact columns using silica 20-30 as shown in FIG. 7. Note the concave CaCO₃ cement where a sand particle was bonded.

FIG. 9. Image from the same mix & compact columns using silica 20-30 as shown in FIG. 7. These are silica sand particles covered with a growing CaCO₃ layer that forms cement at the inter-particle contact. The large particles to the left appear to be well-developed CaCO and possibly evaporates (far left).

FIG. 10. Low-resolution image from a 4″×12″ PVC column #2 using silica 20-30 with bentonite; note the scale (10 μm). These are growing CaCO₃ crystals covering silica sand particles (not seen). The CaCO₃ is randomly covered or associated with clumps of bentonite, which may be serving as points of nucleation.

FIG. 11. Image from the 4″×12″ PVC column #1 using silica 20-30. These are growing CaCO₃ crystals covering silica sand particles. The relatively smooth concave feature is an inter-particle cementation contact.

FIG. 12. Image from the same 4″×12″ PVC column #1 using silica 20-30 as FIG. 11, but zoomed-in to the relatively smooth concave inter-particle feature described above; these are CaCO₃ crystals covering silica sand particles.

FIG. 13. Image from the same 4″×12″ PVC column #1 using silica. 20-30 as in FIG. 11. These are CaCO₃ crystals covering silica sand particles.

FIG. 14. Image from the same 4″×12″ PVC column #1 using; silica 20-30 as in FIG. 11.

FIG. 15. Image from the 4″×12″ PVC column #3 using silica F-60. These are silica sand particles variously covered with CaCO₃ crystals.

FIG. 16. Image from the same 4″×12″ PVC column #3 using silica F-60 as in FIG. 15, but at higher magnification. Note the inter-particle cementation.

FIG. 17. Image from the same 4″×12″ PVC column #3 using silica F-60 as in FIG. 15, but showing different location. These are silica sand particles covered with CaCO₃ crystals.

FIG. 18. Image from the same 4″×12″ PVC column #3 using silica F-60 as in FIG. 15, but at higher magnification. These are silica sand particles covered with CaCO₃ crystals.

FIG. 19. Image from the same 4″×12″ PVC column #3 using silica F-60 as in FIG. 15, but at lower magnification. These are silica sand particles covered with CaCO₃ crystals.

FIG. 20. An image from a columnar EICP study using dry sand injected with cementation solution

FIG. 21. An image from a columnar EICP study using wet sand injected with cementation solution

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular forms “a”, n and “the” include plural referents unless the context clearly dictates otherwise. “And” as used herein is interchangeably used with “or” unless expressly stated otherwise.

All embodiments of any aspect of the invention can be used in combination, unless the context clearly dictates otherwise.

In a first aspect, the present invention provides mineral precipitation method, comprising

(a) combining a porous starting material with

-   -   (i) a source of urease;     -   (ii) urea; and     -   (iii) a source of divalent cations;

wherein (i), (ii), (and (iii) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate mineral precipitation within the starting material to produce a starting material complex; and

(b) contacting the starting material complex with a base under conditions to mitigate formation of ammonium salts in the starting material complex.

In a soil cementation process based upon carbonate precipitation via hydrolysis of urea, adding a base (such as sodium hydroxide) can mitigate the formation of ammonium salts, inhibit nitrification of ammonia (which can lower the pH and start dissolving CaCO₃), and facilitate carbonate precipitation and cementation. It is applicable to work on enzyme-induced calcite precipitation (EICP) as described herein, as well as microbially induced calcite precipitation (MICP). The use of the base helps drive a shift of ammonium→ammonia to limit the potential of reaction reversal due to the ammonium, and will temporarily increase pH near the point of introduction, which also has a beneficial effect with respect to inducing carbonate precipitation. This not only limits the potential for reversal of and enhances carbonate precipitation, but also helps rid the system of toxic ammonium salt by-products. A further effect of adding a strong base to the system is that it may help expose/dissolve some of the soil particle or substances clothing the soil particle and thereby facilitate more and/or stronger bonding of the soil to be improved (e.g. gravel, sand, or silt particles susceptible to degradation/dissolution via base) with the precipitated calcite (calcium carbonate).

Induced carbonate precipitation can enhance the stiffness, strength, and liquefaction resistance of soil. The methods provide an alternative to commonly used soil improvement techniques such as deep soil mixing, stone columns, penetration and compaction grouting, and rammed aggregate piers.

The methods can be used, for example, in stabilizing slopes, improving the bearing capacity of foundations, reducing settlement potential of foundations and embankments, increasing the lateral resistance of foundations, reducing the potential for triggering of earthquake-induced liquefaction; reducing the potential for triggering static liquefaction, mitigating the potential for damaging ground displacements subsequent to triggering of liquefaction; enhancing the stability of slopes and embankments, reducing lateral earth pressures on retaining walls, increasing the passive resistance of retaining walls, increasing the capacity of ground anchors and soil nails, increasing the side resistance and tip resistance of deep foundations, facilitating tunneling in running or flowing ground (dry or saturated cohesionless soil), stabilizing the bottom of excavations, soil erosion control, groundwater control.

Any suitable base may be used that serves to mitigate formation of ammonium salts in the starting material complex. In one non-limiting embodiment, the base is selected from the group consisting of NaOH, KOH, Ca(OH)₂, Mg(OH)₂, and combinations thereof. The amount of base needed is that amount which shifts the acid-base equilibria towards the basic side (i.e., higher pH); this amount will depend on the concentration and the amount of concentrations of urea and divalent cations added to the starting material (or in the starting material complex) used, as will be understood by those of skill in the art. In addition, soil conditions may favor more or less base as certain soils lack buffering capacity and other soils provide some level of buffering, respectively. The goal is to add a sufficient amount of base to shift the acid-base equilibria of the chemical reaction matrix to a high pH (preferably pH>10) which would greatly favor ammonia over ammonium. Thus, as will be understood by those of skill in the art, a lesser amount of stronger base (e.g., higher Molarity or dibasic base) can be used, or a greater amount of a weaker base (e.g., lower Molarity or monbasic). The exact amount of base added will depend on the concentration of ammonium present in the reacting system, or the amount expected to be formed in the reacting system.

In one embodiment, the base is contacted with the starting material complex after most of the carbonate precipitation reaction has occurred, or after nearly all the urea has been hydrolyzed; these are two different reactions, and even if most to all the urea is depleted, carbonate precipitation may still be occurring and can be further “stimulated” or driven by addition of base—again, by shifting the system to more basic conditions which favor mineral precipitation. The timing of contacting can be determined by those of skill in the art based on the intended use. In one non-limiting example, one could draw a sample and test concentrations of either calcium or urea or both and then estimate the extent of the reaction and add base accordingly. In another embodiment the pH can be checked at appropriate intervals and base added as necessary for the intended use. In various further embodiments, the method may comprise checking the pH of the complex at a location of interest to estimate or determine the ammonia-ammonium concentrations, including but not limited to checking Ca, NH₃ and/or NH₄ levels, which can be used to estimate/determine the ammonia-ammonium ratios, and/or estimate/determine the extent/amount of carbonate precipitation.

The contacting; can be carried out for as long as appropriate for an intended use. As will be understood by those of skill in the art, upon contacting of the complex with the base, the reaction and shift from ammonium to ammonia occurs very rapidly, but may be spatially limited to areas near a site of contacting the complex with the base. Thus, in one embodiment, the methods may comprise introducing the base into the complex at more than one location in the complex. Any suitable number of different locations for contacting of the complex with the base can be used, as deemed most appropriate for an intended use.

The base can be contacted with the complex via any suitable technique, including but not limited to injection or percolation of the base into the starting complex, or via any other means of introducing the base into the complex.

“Carbonate mineral precipitation” means mineral precipitates that may include one or more cations that may produce one of several phases of carbonate minerals, including but not limited to calcite. In a preferred embodiment, calcium carbonate precipitates form cementation bonds at inter-particle contacts in the starting material and fill in void spaces in the starting materials (thereby increasing the tendency of the starting material to dilate, or expand in volume, when sheared), and/or cementation of adjacent particles of the starting material.

Several divalent cations, primarily alkaline earth metals and other divalent-state metals (including but not limited to such as calcium, magnesium, strontium, iron, cadmium, manganese, lead, or zinc ions, or combinations thereof), that satisfy the crystalline structure constraints of calcite or calcium mineral carbonates can be used to precipitate carbonate minerals in the present methods. Any suitable source of divalent cations can be used, including but not limited to salts of organic and inorganic compounds such as nitrates, nitrites, chlorides, sulfates, oxides, acetates, silicates, oxalates or mixtures thereof.

The appropriate amount of ions can be determined by one of skill in the art based on the teachings herein; factors to be considered in determining an appropriate amount of ions include, but are not limited the required amount of carbonate precipitate required for the particular application as determined on a stoichiometric basis. In one non-limiting example, if 100 grams (approximately 1 mole) of calcium carbonate (CaCO₃) is desired, then 1 mole of urea (NH₂)₂CO) and at least 1 mole of calcium (Ca²⁺) are required (in addition to alkalinity, the urea also provides the necessary 1 mole of carbon to ultimately form CO₃ ²⁻).

Any suitable starting material, or combinations of starting materials, may be used in the methods of the invention, such as those having a particulate structure or those consisting of discrete soil particles forming a stable framework (or skeleton) or relatively impervious blocks delineated by an interconnected network of fractures or fissures. In a preferred embodiment, the starting material may be unconsolidated or partially consolidated particulate material such as sand, silt, soil, clay, sediments, sawdust or other material that is amenable to in situ cementation, or combinations thereof. As used herein, “particulate” starting materials are starting materials comprising separate particles.

In another embodiment, the starting material may comprise fractured rock masses. Such starting materials may be mixed in situ or mixed and compacted with the urease, the urea, and the source of divalent cations. In a further embodiment, the starting material is saturated.

In further embodiments, the starting material may be gravel, igneous, metamorphic, or sedimentary rocks including but not limited to conglomerate, breccia, sandstone, siltstone, shale, limestone, gypsum, and dolostone, or combinations thereof. In one preferred embodiment, the starting material comprises sand. In another preferred embodiment, the starting material comprises silt. In a further preferred embodiment, the starting material is fractured crystalline rock or cracked concrete.

The starting material may be “porous” in that it enables sufficient passage of the urease, the urea, and/or the source of calcium or other ions and constituents including, but not limited to, buffers and stabilizers, to enable carbonate precipitation with or without cementation. In some applications, the method may work simply by filling the void spaces (e.g. soil pores, rock fractures) with the carbonate precipitate, without any actual cementation of/bonding to the host material.

The components can be combined in any way suitable in light of the specific starling material, the amount of starting material, the components to be used, etc. In various embodiments, the starting material and components are combined by a technique selected from the group consisting of flushing, injecting, mixing, spraying, dripping or trickling onto or into the starting material. The starting material may also be immersed in one or more ways as described above. In addition, secondary non-specific methods may be employed to facilitate carbonate precipitation including, but not limited to, moisture control measures, crystal seeding, and initiation of nucleation sites. In one embodiment, the methods comprise mixing powdered urease or urease in solution with a particulate starting material prior to percolation of a solution comprising the urea and the divalent ion source or combining the urease, urea, and divalent ion in solution with the starting material, mixing them with in situ or mixing them ex situ and then placing or compacting the mixture. In another embodiment, the combining comprises injection of the urease, urea, and/or divalent cations into the starting material via one or more central porous injection tubes, such as are described in the examples that follow. Any number of such injection tubes may be used, depending on the size and depth of the starting material to be treated, among other factors, and any design or size of such injection tubes may be used. In one non-limiting embodiment, field injection tubes may comprise 2 inch or 3 inch diameter perforated pipe that is vibrated or pushed into the soil. The tubes may be placed at varying depths and orientations in the starting material. In one embodiment, injection occurs all along the length of the tube at once, or may comprise injection over small intervals (such as 1-3 foot intervals) working from one end of the tube, which can be accomplished, for example, by sealing off sections where injection is to occur with packers, as in known in grouting technology.

The source of urease may comprise isolated urease or cells expressing urease.

As used herein, “isolated urease” is urease that is extracted from cells and cellular materials. The urease may be synthetically produced or obtained by extraction from any suitable source, including but not limited to bacteria, plants, invertebrates, and fungi. In one non-limiting embodiment, a plant derived urease extract can be used. For example, urease activity is present in various plant leaves and this activity can be realized using crude extracts of the leaves, or isolated enzyme. Urease enzyme as discussed herein is characterized by the reaction it catalyzes and identified by EC 3.5.1.5 (i.e. Reaction: urea+H₂O═CO₂+2 NH₃). In one embodiment, the urease enzyme is isolated from the jack-bean plant (SEQ ID NO:1). The amino acid sequences of exemplary ureases for use with the present invention are provided below. However, it will be clear to those of skill in the art that any enzyme identified by EC 3.5.1.5 can be used in the methods of the invention, including but not limited to a urease comprising or consisting of any one of SEQ ID NOS: 2-5, where SEQ ID NO:2 is a soybean urease, SEQ ID NO:3 is a Agaricus bisporus urease, SEQ ID NO:4 is a Schizosaccharomyces pompe (strain 972/ATCC 24843) urease, SEQ ID NO:5 is a Sporosarcina pasteurii urease, and SEQ ID NO:6 is a Pseudomonas syringae (strain R728a, urease.

The appropriate amount of urease needed can be determined by one of skill in the art based on the teachings herein; factors to be considered in determining an appropriate amount of urease include, but are not limited to:

(a) urease source type (e.g. Jack bean vs. other source, such as a cell expressing urease)

(b) urease purity, which dictates enzymatic activity (i.e. rate of conversion of urea to products); and

(c) the stability/half-life of the enzyme matrix used, where the “enzyme matrix” refers to the specific form of the enzyme mixture used such as liquid, powder and/or solid when combined with or used apart from stabilizers, buffers, fillers or other media to facilitate its desired use.

For example, assuming that for practical purposes that transport via diffusion, advection and dispersion is not limiting the availability of urea or calcium to the enzymes—or vice versa—(e.g. we thoroughly mix the soil and cementation constituents or we actively pump the cementation constituents into the soil or do something to assure that the right constituents get to where they need to be), then in a homogenous soil (i.e. without zones of blocked flow or disproportionately high/preferential flow) we could expect an approximately linear relationship between urea conversion and required amount(s) of enzyme needed to convert “x” grams of urea to “y” grams of calcium carbonate over a given time frame. This is also dependent on the on the amount of calcium ions available for precipitation. A sufficiently high concentration of calcium to form calcium carbonate is needed, with hydrolysis of urea just one part of the overall process. If a soil mass requires a total amount “x” grams of urea to be converted into products for calcium carbonate formation, and “y” grams of enzyme can only convert 50% of “x” during its functional life time, then theoretically twice as much enzyme is needed to fully convert “x” grams of urea.

Urea is an organic compound of the chemical formula CO(NH₂)₂. Urea is a colorless, odorless, highly water soluble substance with very low toxicity (LD50=12 g/kg for mouse, Agrium MSDS), and is widely commercially available. Any suitable source of urea can be used, including but not limited to those disclosed herein.

The appropriate amount of urea can be determined by one of skill in the art based on the teachings herein: factors to be considered in determining an appropriate amount of urea include, but are not limited, to the amount of carbonate required for the particular application and the desired increase in pH and alkalinity as determined on a stoichiometric basis. As will be understood by those of skill in the art, the amount of carbonate precipitation required varies from one use to another.

It will be understood by those of skill in the art that the step of “combining” the starting material with effective amounts of urease, urea, and cations covers any process that results in the bringing together of the three constituents in a manner that results in precipitation of carbonate minerals in the starting material. The reactants may be added to the starting material simultaneously or sequentially. For example, there may be applications where one or two of the constituents are already present in the starting material, in which case the step of “combining” will involve the addition of only the missing components. In one embodiment, the urea and cations are admixed and then added to the urease prior to application to the starting material. However, it will be appreciated by those of skill in the art that the constituents may be combined in other ways to carry out the method of the invention.

By manipulating the relative effective amounts of the various components, the methods of the present invention enable the user to control carbonate precipitation by controlling the amount of carbonate formed and the rate at which it is formed. This flexibility means the methods of the present invention can be used in a wide range of applications from those that require a reasonably modest increase in the strength, stiffness, or dilatancy or modest decrease in the permeability in the starting material to those that require larger changes in the relevant property. As used herein, “dilatancy” refers to the tendency of a soil to expand in volume during shear. This is an important property, for example, with respect to reducing liquefaction potential.

The effective amounts of the various reactants combined according to the method of the present invention may vary depending, at least, on the amount of urease used, the characteristics of the starting material and the conditions under which precipitation is to occur, the desired final strength, stiffness, dilatancy, or permeability of the treated porous material and the amounts of the other reactants in the reaction mix. The present application enables those of skill in the art to determine the relative amounts of the various reactants required for a given application and to apply the method to various starting materials and for a variety of end uses. The method of the present invention may be adapted to allow for the rate of mineral precipitation to be controlled, as required. When rapid or slow formation of the precipitate is desired the amounts and/or relative amounts of the reagents can be selected accordingly to bring about the desired rate of formation. In one non-limiting example, enhancement of the methods may comprise providing stronger nucleation sites on particles of the starting material by high-pH pretreatment of the particles of the starting material to be improved.

Depending on the requirements of a particular application or mode of use of the present invention, rapid formation of the precipitate may be required. Alternatively it may be preferred for the precipitate to be formed slowly. Based on the teachings herein, those of skill in the art will be able to modify the protocol to attain faster or slower formation of the precipitate.

The methods of the invention may be repeated (once, twice, three times, or more) in order to attain the desired amount of mineral precipitate strength, stiffness increase, or permeability reduction. When the method is repeated to gain incremental increases in strength or stiffness or reduction in permeability, not all of the reagents need to be added each time. For example, residual urease activity may still be sufficient for one or more subsequent rounds of the method. A skilled person is readily able to determine the particular amounts of reagents required for use in subsequent rounds of the method of the present invention.

The methods may be applied in situ without disturbing the starting material. This is particularly important for applications where the starting material is delicate or fragile or for other reasons must not be disturbed. Examples include, but are not limited to, when applied in the field where the starting material to be improved (e.g. made resistant to earthquake-induced liquefaction) is underneath or adjacent to an existing structure or facility that is sensitive to ground movement (e.g. settlement or heave).

As will be understood by those of skill in the art, the methods may comprise use of other components as appropriate for a given use. In one embodiment, the methods may further comprise use of a stabilizer (including but not limited to powdered milk) to increase enzyme stability and functional time. The methods can be carried out under any temperature conditions suitable to promote carbonate cementation.

In another embodiment, the method further comprises introducing a clay slurry into the starting material prior to or concurrent with combining the starting material with the urease, the urea, and the source of divalent cations. This embodiment is particularly useful when the starting material comprises a high permeability starting material (e.g. a coarse-grained soil), as it will help to retain the active components in the starting material where cementation is desired; the clay particles may also serve as nucleation points for carbonate precipitation. As used herein, a “clay slurry” is any clay in suspension. In various non-limiting embodiments, the clay may comprise montmorillonite clay (also known as bentonite), attapulgite, or combinations thereof. The amount of clay slurry, the specific amount of clay in the slurry, and the timing/number of times the clay slurry is administered may vary depending, at least, on the characteristics of the starting material and the conditions under which precipitation is to occur, the desired final strength, stiffness, dilatancy, or permeability of the treated porous material and the amounts of the other reactants in the reaction mix. The present application provides examples that enable those of skill in the art to determine the relative amounts of the clay slurry appropriate for a given application and to apply the method to various starting materials and for a variety of end uses.

In a preferred embodiment, the starting material comprises a column of starting material. As used herein, a “column” refers to relatively linear prisms of soil that are to be stiffened and/or strengthened to reinforce the in situ soil mass and/or transfer load to greater depths in the soil stratum. As will be understood by those of skill in the art, the prism of soil extends below a surface of the starting material, in various embodiments, the column is at least 0.6 meters long and 0.15 meters in diameter. In various further embodiments, the column is at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 50, or more meters long/deep. In various further embodiments, the column is at least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, or more meters in diameter. In one exemplary embodiment, for the mitigation of earthquake-induced liquefaction under residential structures, the columns are between about 0.3 meters to 1.5 meters in diameter; more preferably about 0.9 meters in diameter and up to 20 meters long, in this embodiment, the methods comprise combining the starting materials so that carbonate precipitation of the starting material occur at in a radial pattern around an injection tube (e.g. perforated pipe) to form the column. In one embodiment, carbonate precipitation occurs at only one or more specific areas within the column, such as at a specific location where the starting materials are combined within the column (for example, where liquefiable soil strata is intersected by the injection tube). In another embodiment, carbonate precipitation occurs throughout the column.

This embodiment provides further improvements over prior methods, which are focused on improvement of the entire mass of soil in the improvement zone. In this embodiment, the methods permit production of stone-like materials via local cementation. Columns of improved starting materials, such as soils, have broad application in geotechnical practice, including reducing the potential for earthquake-induced liquefaction; mitigating the potential for damaging ground displacements subsequent to earthquake-induced liquefaction, improvement of foundation bearing capacity, support of embankments, slope stabilization, stabilization of the base of excavations, support of underground openings, and a variety of other geotechnical purposes. The limitations associated with bio-plugging can also be mitigated by using this technique to improve columns of soil (rather than the entire soil mass). Such columns of improved starting material possess, for example, increased strength, stiffness, and liquefaction resistance relative to the non-treated starting material. Bio-plugging refers to hindering the mobility of the compounds that were added to the starting material for the purpose of inducing mineral precipitation due the decrease in permeability associated with mineral precipitation, in another embodiment, the methods can be used for fugitive dust control (e.g., wind-blown erosion).

In one non-limiting example, consider a 4.2-meter long, 1-meter diameter column of soil with a porosity of 0.31 is to be improved by precipitating 5% calcium carbonate by weight. This column has a total volume of 3.29 m³ and a pore space of 1.0 m³. Assuming a calcium carbonate density of 2.71 g/cm³(2710 kg/m), approximately 136 kg of CaCO₃ is used to precipitate in the improvement zone (i.e. the column). To obtain 136 kg of CaCO₃ (1,360 moles), 81.6 kg of urea (1,360 moles) and 54.4 kg of calcium (1,360 moles) are used, assuming that all added constituents are available and used in the reaction process to precipitate CaCO₃. Since there are 2 amine groups per urea molecule, 2,720 moles of ammonia nitrogen and 1,360 moles of carbon dioxide are released from 81.6 kg of urea. Assuming a minimum urease activity of 15,000 units/grain urease powder (type 3 urease from Jack bean [Sigma Aldrich]), where one-unit is the release of 1.0 mg ammonia nitrogen from urea in 5 minutes at pH 7.0 at 20° C., then 10 grams of this particular grade of enzyme (150,000 units) will release 150 grams of ammonia nitrogen (8.8 moles) in 5 minutes. Therefore, 26 hours (1546 minutes) are required to fully catalyze the release of carbon dioxide and ammonia from 81.6 kg of urea.

Any suitable technique/configuration for introducing the components into the column may be used as deemed appropriate. For introduction of the urease into columns, injection could be done at one site per column or at multiple sites along the column. If multiple sites, then the concentration may the same or different at the different locations.

In another embodiment, the starting material is saturated. As used herein, “saturated” means that the soil has reached its maximum water content; if any more water is added it will either drain downward, flow upwards, or turn the soil into a suspension wherein there is little to no inter-particle contact. In one example, the saturated starting material is below the water table. When the starting material is below the water table it is usually saturated, unless there is gas in the soil. Saturated zones may also exist above the water table due to capillary rise, with the extent of the saturated zone above the water table depending upon the pore size of the starting material.

In a second aspect, the present invention provides kits comprising

(a) isolated urease;

(b) urea;

(c) a source of divalent cations, preferably calcium ions; and

(d) a base;

wherein (a), (b), (c) and (d) are provided in amounts effective to cause carbonate cementation when combined with a particulate starting material or a starting material complex. In another embodiment, the kit further comprises a clay slurry.

All definitions, embodiments, and combinations thereof of the first aspect apply equally to this second embodiment, unless the context clearly dictates otherwise. Thus, the kits may further contain any of the components or combinations thereof disclosed for use with the methods of the invention, including but not limited to stabilizers, buffers, nucleation aids, etc.

EXAMPLE 1 Carbonate Cementation Via Plant Derived Urease

Methods

Ottawa 20-30 Sand

Laboratory column tests were conducted using plant derived urease to induce calcium carbonate (CaCO₃) precipitation in Ottawa 20-30 sand These tests were carried out in 6″×2″ (152 mm×51 mm) acrylic tubes and membrane-lined 2.8″×6″ (71 mm×152 mm) split molds (for creating specimens for triaxial testing). Three acrylic tubes and two columns for triaxial testing were filled with 20-30 Ottawa silica sand (mean grain size 0.6 mm, coefficient of uniformity 1.1) and treated as follows: tube #1: the sand was dry pluviated via funnel at ≈3″ (76 mm) drop height and then received 5 applications of a cementation solution containing urea and calcium chloride mixed with 1.4 g/L enzyme (total solution volume ≈300 ml); tube #2: sand was added in same manner as tube #1 and then received 2 applications (≈150 ml total) of the same cementation solution mixed with 1.4 g/L enzyme; tube #3: the lower-third of tube was filled with sand and dry enzyme (≈3 g), the remainder of the tube contained dry pluviated sand without enzyme, and the tube then received 2 applications (≈150 ml) of the cementation fluid with no enzyme added.

Approximately 100 mL of a pH=7.8 solution containing 383 mM urea (reagent grade, Sigma-Aldrich), 272 mM CaCl₂-2H₂O (laboratory grade, Alfa Aesar) was used for the first application in each acrylic tube. Subsequent applications employed approximately 50 mL of a pH=7.6 solution containing 416 mM urea and 289 mM CaCl₂-2H₂O. Solution concentrations, while variable, were formulated within a reasonably similar range as a matter of convenience. In each application, the cementation fluid was poured into the top of the acrylic tube with the bottom closed off. The cementation fluid was allowed to stand, loosely covered, in the acrylic tube for at least 24 hours and then drained out the bottom of the cylinder. The next application followed immediately after drainage was complete. Drainage was accomplished by puncturing the base of the cylinder with a 20-gauge needle. When drainage was complete, the needle was removed and the puncture was plugged with a dab of silicone. Occasionally, the needle became plugged and an additional needle was inserted through the base. The triaxial columns were filled with sand in the same manner as tube 1 and then received 2 applications (each application ≈250 ml) of cementation solution with 1.4 g/L enzyme.

In each application of cementation fluid, the fluid was added until it rose to approximately ½-inch (12-mm) above the soil line. After 2 applications, tubes #2 and #3 were allowed to air dry for several days and then analyzed. Experimentation with tube #1 was continued for several more days as three more batches of cementation fluid were applied. The last 2 applications of cementation fluid were allowed to slowly drain through the needle in the base immediately after application rather than sit for 24 hours (drainage rate ≈10-25 ml/hour). The triaxial columns were allowed to stand for at least a week after the second cementation fluid application and then drained.

After drainage was complete, the triaxial columns were moved to a triaxial testing device. After draining the specimens from the acrylic tubes and after the completion of the triaxial tests, all samples were triple washed with de-ionized water. Tubes #2 and #3 were separated in 3 layers, while tube #1 was separated into six layers (for better resolution). Each layer from the specimens in the acrylic tubes and the entire mass of the triaxial specimens were acid washed to determine CaCO₃ content by oven drying for 48 hours, weighing, digesting with warm 1M HCl, washing, drying, and reweighing to determine carbonate mineral content.

Several of the cemented specimens were analyzed for mineral identification using X-Ray Diffraction (XRD). Samples were ground in an agate mortar and pestle and powdered onto a standard glass slide for analysis. Scanning electron microscopy (SEM) imaging was performed on intact cemented chunks of material with an Agilent 8500 Low-Voltage SEM (LV-SEM). A LV-SEM is a field emission scanning electron microscope capable of imaging insulating materials, such as organic and biological substances without the need for a metal coating and without causing radiation damage to samples.

Ottawa F-60 Sand

A triaxial column was prepared using Ottawa F-60 silica sand (mean grain size 0.275 mm, coefficient of uniformity 1.74) to investigate enzymatic ureolytic CaCO₃ precipitation in a finer grained material than Ottawa 20-30 sand. The specimen was prepared in the same manner as described for the triaxial columns for the Ottawa 20-30 sand. The cementation fluid for the first of the two applications contained approximately 2.0 g/L enzyme, 400 mM urea (reagent grade, Sigma-Aldrich), 300 mM CaCl₂-2H₂O (laboratory grade, BDH) at pH=7.7. The fluid for the second application contained 1 M urea-CaCl₂-2H₂O solution at pH=7.8 without any enzyme. After the test, the triaxial specimen was washed and subject to acid digestion in the same manner as the Ottawa 20-30 triaxial specimens.

Results

Acrylic Tubes

Approximately 100 ml of cementation solution was delivered per application for the first application in each acrylic tube. However, the amount of solution the tube would accept was notably reduced in subsequent applications, when less than 75 ml was typically required to fill the tubes to ≈½ inch (12 mm) above soil line. At the conclusion of the experiment, precipitation was visible along the entire length of tubes 1 and 2. Internally the cementation was variable, with some highly cemented zones and other zones with little to no cementation,

Tube 1 yielded mostly small, loose chunks of sand with strong effervescence upon digestion. Most of this column appeared un-cemented and exhibited unusually viscous behavior when wet. A fairly large (compared to column diameter) piece of strongly cemented sand (no(breakable without tools) formed in the deepest layer of tube 1. Tube 2 had many small chunks of weakly cemented sand with strong effervescence upon digestion. Tube 3 had little to no precipitation in the top layer (i.e. this layer did not show any indication of carbonate upon acid digestion.) The deepest layer of tube 3 contained many pieces of weakly cemented sand that effervesced strongly upon digestion. The middle layer of tube 3 contained a few pieces of cemented sand that effervesced moderately upon digestion. The results from the acid washing are presented in Table 1.

TABLE 1 Results from Experiment Set 1 using 20-30 Ottawa silica sand Summary of Results Weight Amt. Total Theor. Change of Amt. Max via CaCO₃ CaCO₃ CaCO₃ Tube # Layer Digestion (g) (g) (g) 1 1  11% 3.57 11.8 ≈14.5 2 3.8% 1.67 3 2.7% 1.73 4 2.1% 1.40 5 2.3% 1.74 6 2.0% 1.64 2 1 0.76%  0.63 2.07 ≈4.35 2 0.65%  0.69 3 0.49%  0.75 3 1 0.23%  0.31 3.57 ≈4.35 2 0.58%  0.63 3 1.7% 2.63

The theoretical maximum CaCO₃ content s the stoichiometric maximum balanced on initial concentrations. The primary experimental differences between the tests are (1) the number of applications of cementation fluid and (2) the manner in which the urease was delivered. The results indicate that there is greater carbonate precipitation with increasing number of applications, as expected. The data show more precipitation in (or on) the top layer of tubes 1 and 2 but not in tube 3, as the enzyme was physically confined to the lower-third layer in tube 3 during sample preparation. In the top layer of tube 3, where no urease was mixed with the sand, carbonate precipitation was nearly undetectable. There was no visual evidence of precipitation and practically no measurable change in weight of this layer after acidification (weight change=0.23%). In the bottom layer of tube 3, where 3 g of dry enzyme was mixed with the soil, there was a weight change of 1.7% following acid washing. The middle layer of this specimen had a Minor change in weight (0.58%), possibly due to uneven distribution of the layers during preparation or splitting of the specimen or to upward migration of urease from the bottom layer. XRD analysis confirms that calcite is the mineral phase present in the cemented soil chunks. LV-SEM images, presented in FIG. 1, show silica (quartz) sand particles cemented with calcium carbonate and various morphological features associated with the cementation process on the silica surface.

Triaxial Columns

The three triaxial sand columns (2 Ottawa 20-30 sand columns and 1 Ottawa F-60 sand column) were tested in drained triaxial compression prior to acid digestion. All three columns were able to stand upright after removal of the split mold. The results of the triaxial compression tests performed on the 20-30 Ottawa sand are presented in FIG. 2 and the results for the F-60 Ottawa sand are presented in FIG. 4. The carbonate cement content for one of the 20-30 silica sand columns was 2.0% CaCO₃ (by weight). The carbonate content of the other 20-30 Ottawa sand column could not be quantified due to unintended sample loss. The carbonate cement content for the finer grained F-60 Ottawa sand was 1.6% CaCO₃ (by weight). The results show substantial strength increase for all 3 sand columns tested.

Conclusion

Sand column tests have shown that agriculturally-derived urease can be used to induce calcium carbonate precipitation in sand. Sand columns were developed using Ottawa 20-30 and F-60 sand and three different preparation methods: dry pluviation followed by percolation of a calcium-urease-urea cementation solution, pluviation into a calcium-urease-urea cementation solution, and mixing the sand with urease prior to pluviation with a calcium-urea solution. Cementation was observed in all of the columns. XRD and SEM testing confirmed that calcium carbonate (specifically calcite) was the cementing agent. Acid digestion showed that increased applications yielded correspondingly greater carbonate precipitation. The quality of cementation, as determined by the effort needed to break apart cemented chunks of sand, varied depending on the sampling location within the column. Triaxial test results on cemented columns showed substantial strength increase over non-cemented columns at the same relative density.

EXAMPLE 2

Methods

Three sand columns were constructed in 12″ long by 4″ (I.D.) clear PVC tubes (schedule 40) and labeled “Column #1,” “Column #2,” and “Column #3.” Column #1 and Column #2 used Ottawa 20-30 sand, a medium (grain size) sand, while Column 3 used Ottawa F60 sand, a fine sand. Each PVC column was closed-off on one end using a flexible cap (Qwik Cap) and fastened with a hose clamp. The 3 columns were filled with densified sand to depth of approximately 4″. Next, an injection tube was made of flexible ¼″ (O.D.) Tygon tubing with 6-8 needle size holes (18 gauge) in a radial pattern along the last 1.5″ of the tubing. The injection tube was then placed along the axis of the column with the end containing needle holes approximately 0.25″ above the 4″ densified soil layer. The columns were then loosely filled with the designated sand using a small scoop to a height of 10″ (i.e., 6″ above the 4″ base layer). Each column was then filled through the injection tube with approximately 700 mL of tap water at 35° C. in order to fully hydrate the columns—the final water level was just above the top of the soil (i.e. slightly more than 10″ from the bottom of the tube). In order to create a uniform soil column, the columns were than densified by firmly tapping the outside of the PVC tube in a radial pattern using a blunt instrument, which reduced the final soil column height from 10″ to approximately 9.4″ in each column Approximately 40 mL of bentonite slurry was injected into Column #2 through the Tygon injection tube. Each column then received 155 mL of the reaction medium consisting of tap water with 1.36 M urea and 0.765 M calcium chloride dehydrate (pH=7.3, 35° C.). After receiving the reaction medium, the injection tubes were flushed with 2 mL water. Then 15-20 mL of enzyme solution consisting of 0.44 g/L urease enzyme (Sigma Aldrich Type-III, Jack Bean Urease, 26, 100 U/g activity) and 4 g/L stabilizer (nonfat powdered milk) were introduced to each column through the injection tube. Finally, another 3-4 ml, of reaction medium was injected into each column followed by a 2 mL water flush. The injection tubes were closed with polypropylene pinch clamps and the columns were capped with plastic clear wrap and placed in dark, warm (30° C.) environment for 2.8 days. On day 7, an additional 115 mL of reaction medium and 10 mL of enzyme solution was delivered to each column in a manner similar to described above. Approximately 3 ml (0.1% v/v) of 1M NaOH was injected into each column 48 hours before the experiment terminated.

Results:

Upon disassembly, all 3 columns showed strong carbonate cementation in a cylindrical zone around the end of the inject tube. The results varied slightly for each column, as follows:

Column #1 (20-30 medium sand)—A region of strongly cemented soil began ≈2.5″ from the column bottom. The cemented zone was ≈4.5″ in length and displayed a prominent rounding at the top and a flat surface at the bottom. Overall, the cemented region appeared to have a cylindrical bottom and bulb-shaped upper portion (FIG. 4). Several chunks of cemented soil were dislodged to access the most strongly cemented region which had the injection tube firmly embedded. The cemented region could not be dislodged from the PVC column without the use of hand tools.

Column #2 (20-30 medium sand w/40 mL bentonite slurry)—A cylindrical region of strongly cemented soil began ≈3″ from the column bottom, was ≈4″ in length and displayed small rounding near the top and a flat surface at the bottom. Overall, the cemented region was mostly cylindrical (FIG. 5). Several chunks of cemented soil were dislodged to access the most strongly cemented region which had the injection tube loosely embedded. The cemented region could not be dislodged from the PVC column without the use of hand tools.

Column #3 (F-60 fine sand)—A region of strongly cemented soil began ≈3″ from the column bottom, was ≈2.5″ in length and displayed a clear bell-shaped top and a flat surface at the bottom. Overall, the cemented region appeared bell-shaped (FIG. 6). The entire soil column was dislodged upon disassembly and many chunks of cemented soil were dislodged to access the most strongly cemented region which had the injection tube firmly embedded. In addition, the soil overall mass contained many small (1-3 mm) pieces of cemented sand. The column had a strong smell of ammonia and displayed significant amounts of gas bubbles during washing.

Example 2 demonstrates (for example) the ability to create roughly cylindrical columns in a saturated soil (i.e. below the water table); stabilize a fine grained material; and the optional facilitation of cementation by first injecting a bentonite slurry into the column, which provides particular benefit when used with more porous (coarser) starting materials.

EXAMPLE 3

In one experiment, using 4″ by 12″ PVC columns, a final step was used as follows: the injection of approximately 0.01% by volume (˜2 ml/column) of pH=13 NaOH during the last week of the experiment. The purpose of this injection was to help drive a shift of ammonium→ammonia to limit the potential of reaction reversal due to the ammonium (this will temporarily increase pH near the point of injection, which is also has a beneficial effect with respect to inducing carbonate precipitation)—this not only limits the potential for reversal of and enhances carbonate precipitation, but also helps rid the system of toxic ammonium salt by-products. A further effect of adding this strong base (NaOH) to the system is that it may help expose/dissolve some of the soil particle or substances clothing the soil particle and thereby facilitate more and/or stronger bonding of the soil to be improved (e.g. gravel, sand, or silt particles susceptible to degradation/dissolution via base) with the precipitated calcite (calcium carbonate).

EXAMPLE 4 Columnar EICP Stabilization Tests

The details of chemical and enzyme preparation are the same as those described in the previous examples, as is the application to different soil types. Test were set-up in 5-gallon buckets using coarse sand. Perforated PVC tubes were placed in center of 5-gal buckets. Buckets then filled w/coarse silica sand; half were wet studies (the soil was essentially saturated, i.e. there was water to almost the top of the soil layer at the start of the experiment) and half were dry studies (i.e.: dry soil). The wet tests simulate improving soil below the water table while the dry test simulates soil above the water table.

Cementation solution (same conditions as disclosed above) was poured into the perforated PVC tubes. Approximately 3.5 weeks later, 0.1% NaOH (1M) added and the mix was allowed to sit for approximately 2 weeks. The buckets were than rinsed & drained and allowed to sit for approximately 1 week. Cemented soil specimens were then removed from the buckets for analysis. See FIGS. 20-22. The soil stuck to the tubes in all experiments (wet or dry), while the strength of attachment varied from firmly stuck to weakly stuck based on the extent of cementation near the tube. In general, the larger and more uniform the cemented specimen, the more likely to be stuck to the injection tube; similarly, cementation in wet soil generally resulted in some of the cemented specimen being stuck to the injection tube. Penetration of the cementation solution into the dry soil was somewhat more uniform in the wet soil. These data demonstrate effective cementation both wet and dry soil. These data demonstrated that the addition of base (1) resulted in a shift towards 1111₁ and away from NH₄ ⁺, (2) inhibited nitrification, and (3) shifted the CaCO₃ reaction towards further precipitation. 

We claim:
 1. A cementation method, comprising: (a) combining sand with (i) agriculturally-derived urease; (ii) urea; and (iii) calcium ions; wherein (i), (ii), (and (iii) are provided in amounts effective and the combining is carried out under conditions suitable to cause carbonate mineral precipitation of the sand to produce a starting material complex, and wherein the combining comprises: mixing the agriculturally-derived urease with the sand; and injecting into the sand, under pressure and via an injection tube, a solution comprising the urea and the calcium ions into the mixture of sand and agriculturally-derived urease; (b) utilizing powdered milk as a stabilizer; and (c) injecting into the starting material complex, under pressure and via an injection tube, Ca(OH)₂ under conditions to mitigate formation of ammonium salts in the starting material complex.
 2. The method of claim 1, wherein the Ca(OH)₂ is injected under pressure into the starting material complex at a plurality of locations.
 3. The method of claim 1, wherein the pH of the starting material complex increases where the starting material complex contacts the Ca(OH)₂.
 4. The method of claim 1, wherein the method is used for improving bearing capacity of a building foundation.
 5. The method of claim 1, wherein the combining is conducted underneath or adjacent to an existing structure that is sensitive to ground settlement or heave.
 6. The method of claim 5, wherein the carbonate mineral precipitation does not cause ground settlement or heave affecting the existing structure.
 7. The method of claim 6, wherein step (a), step (h), and step (c) are carried out in situ without disturbing the sand.
 8. The method of claim 1, wherein the injection tube comprises a perforated pipe.
 9. The method of claim 1, further comprising introducing a clay slurry into the sand prior to or concurrent with combining the sand with the agriculturally-derived urease, the urea, and the calcium ions.
 10. The method of claim 1, wherein the sand is configured in a column beneath a residential structure, wherein the injecting provides mitigation of earthquake-induced liquefaction damage to the structure, wherein the column has a diameter of between 0.3 meters and 1.5 meters, and wherein the column has a length of less than 20 meters.
 11. The method of claim 1, wherein the sand is saturated with water.
 12. The method of claim 1, wherein the sand is not part of an oil-bearing formation.
 13. The method of claim 1, wherein the performing step (a), step (b), and step (c) results in a reduced liquefaction potential of the sand. 